1. Technical Field of the Invention
The present invention generally relates to a memory device into which data from the preceding stage is written and from which data is read depending on the processing status of the subsequent stage, and more particularly to a memory device of which a required memory capacity is split into a plurality of split memories for use in processing.
2. Description of the Related Art
Conventionally, encoding and compressing processing have been performed in recording and transmitting of audio data or image data, and decompressing and decoding processing have been performed in reproducing and receiving.
For example, in recording an audio signal in an MD (minidisc), the audio signal is first converted into digital delta (A/D conversion), and in turn the digital data is split into three bands (frequency bands) comprising a low, an intermediate, and a high band by a digital filter called a QMF (Quadrature Mirror Filter), and subjected to MDCT (Modified Discrete Cosine Transform) processing, quantization processing, and the like, and the resultant data is written into the MD. Accordingly, reproduction of such data involves reverse processing such as dequantization, IMDCT (Inverse MDCT) processing, inverse filtering processing by IQMF (Inverse QMF), D/A conversion, and the like.
When performing such processing, a memory is required to store the data being processed in the process of the processing. For example, when the MDCT processing is performed, data fed from the QMF at the preceding stage is processed, wherein L channel data and R channel data are alternately fed from the QFM in time series. On the other hand, in the MDCT processing, the L channel data and the R channel data are separately processed.
Accordingly, data required for the processing is kept stored, and at the stage when the data to be processed is all available, the processing is started. Thus, the processing requires a memory for storing the data to be processed and for storing the data inputted thereto while the processing is being performed. Further, calculations associated with the MDCT involve recursive operations, thus also requiring a memory for storing data being processed.
For the MDCT processing, a large quantity of data is to be stored as mentioned above, causing a problem that a large capacity memory (generally SRAM) is required.
Besides, in the MDCT, data is split into SGs (Sound Group) each having a period of 11.6 msec for processing. At a boundary of two SGs, data is used overlapped to a degree by both SGs. Therefore, the overlapped data is used in the MDCT relating to the two SGs. For this reason, the data used twice must be prevented from being overwritten by the following data.